I agree that this is a problem... but I think that taming the creature should mark as a failure of the quest. The quests specifically asks you to kill the creature not control the creature for your own ends. It works out neatly, either you get harridan or you get the spearmen. I consider either option a win-win. The taming and getting the spearmen I consder way to much.

I play with 'Beastlord' as a profession, so I have Tame as a spell.  Yes, there is a collar that also gives you a similar skill, but ... it can be dangerously unreliable if the collar isn't used by someone with a high Spell Mastery because Tame (whether as a spell or from the collar) can only be cast/used immediately adjacent to the beast, and if the beast resists, you cannot make another attempt on them -- and they likely get another turn with your sovereign/champion immediately adjacent to them.

The three sons is the dying mother wanting you to kill the beast. I think it should really be that if you tame the beast the three sons should attack you instead to avenge their dying mother. I just think the quest would be more dynamic with these outcomes. Where choices in battle change results in the quest.

The rats quest you find a corpse spider instead of harridan.